1. Field of the Invention
The present invention relates to the control of stepper motors and more specifically to a method for reducing the number of components required to control a stepper motor.
2. Description of the Prior Art
Stepper motors are well suited for use with digital control circuitry. In previous years, the control circuitry for driving a stepper motor required a fully populated board of analog, MSI and SSI integrated circuits. Needless to say, the manufacturing cost of such controller boards was quite high and they consumed significant amounts of space.
Dedicated controller chips have recently been introduced to replace some features found on the earlier full board controllers. One such controller chip, designated the PBL 3770, is described in an application note of the RIFA Integrated Circuits Company, dated April 1986. FIG. 1 is a schematic of a stepper motor controller 5 as shown in the RIFA application note. The illustrated circuit 5 has the disadvantage of requiring two separate controller chips, 6 and 7; one for each winding of the stepper motor 8. A plurality of control signals (I0A, I1A, I0B, I1B, Phase A, Phase B) need to be continuously supplied from some external command device(s) (not shown) for proper motor operation. The PBL 3770 is designed to be compatible with LS-TTL logic and is accordingly implemented using bipolar transistors. Bipolar designs consume significant power in comparison to control circuitry based on CMOS technology. Bipolar designs are also more susceptible to noise problems than are CMOS based control circuits. A large number of external components including a plurality of discrete capacitors 9, diodes 10 and resistors 11 are required for each of the stepper controller chips, 6 and 7, to complete the motor controller circuit 5. These additional components 9, 10 and 11, together with the command signal generating device(s) (not shown), add to the cost and complexity of a stepper motor control system.
Unitrode provides a single chip, designated the PIC-930, which is substantially the equivalent of two RIFA PBL3717 chips in one package (the PBL3717 is an earlier version of the PBL3770). The Unitrode chip reduces the component count of a controller circuit some what but it does not overcome the problem of requiring external command devices. The component count of a complete controller system is therefore still elatively high and circuit complexity is not substantially reduced. The cost, complexity, and multiple command requirements of designs using dedicated control IC's such as the PBL3770 and PIC930 represent and inconvenience to users. This inconvenience tends to detract from the general acceptance of stepper motors in electromechanical applications for which they are otherwise well suited.